Fuel control valve systems for supplying fuel to aircraft are well known. Typically, such valve systems include a surge control servo system for preventing any spike or high frequency change in the flow rate from the system pump or from quick tank closure being transmitted to the aircraft fuel intake. A manually operated deadman control switch is also used. The deadman switch only allows fuel delivery to the aircraft while an operator manually actuates the deadman switch. The above features of fuel control systems are used to prevent fuel spillage during a refueling operation.
FIG. 6 is a cross sectional view of a conventional flow control-surge control servo system 600, with the main flow control valve 110 closed. Valve system 600 includes three main components: the main flow control valve 110, the surge control servo 650, and the pressure control valve 170, all within a housing 102.
Main flow control valve 110 receives fluid at an inlet 116 and transmits the fluid to an outlet 124. Main flow control valve 110 has a poppet 112. Poppet 112 has a front face 112a and a rear face 112b, which faces a chamber 122. A main bias spring 118 engages rear face 112b and biases poppet 112 towards its seat 114. Poppet 112 transmits fluid to the outlet 124 when poppet 112 is open. When the pressure force on front face 112a is greater than the sum of the bias force of spring 118 and the fluid pressure force on rear face 112b, poppet 112 opens. When the pressure force on front face 112a is less than the sum of the bias force of spring 118 and the fluid pressure force on rear face 112b, poppet 112 closes. The outlet 659 of main flow control valve cavity 122 is closed by surge piston 654 from inlet pressure 126.
Housing 102 has a cylindrical bore 104 having an inner wall, in which surge control 650 is slidably mounted. Surge control 650 includes first and second pistons 648 and 654, respectively, and a connecting member 652 between pistons 648 and 654. A top chamber 657 is formed between the first and second pistons 648 and 654. Chamber 657 has a load sense port 644 for receiving fluid at a load sense pressure controlled by the load sense signal. The load sense pressure signal is also transmitted from chamber 657 to a load sense chamber 181 of pressure control 170, via a fluid path 668.
Surge control servo 650 has a biasing spring 651 positioned in chamber 657 between the first and second pistons 648 and 654. The bottom end of spring 651 is fixed in place against end of bore 104. The top end of spring 651 exerts a bias force on the bottom of first piston 648, in a direction tending to open outlet 659 to inlet pressure 126 to fill cavity 122 causing valve to close.
A load sense fluid pressure signal is provided at a load sense port 644. Surge control servo 650 moves between a closed position (not shown in FIG. 6), in which piston 654 blocks passage 126 and an open position (shown in FIG. 6). Under normal operating conditions, surge control servo 650 remains in the closed position. As long as the load sense pressure is below a predetermined threshold value, changes in load pressure are accommodated by the pressure control valve assembly 170, described below.
Piston 648 of servo valve 650 separates the load sense chamber 657 from a pressurized air chamber 634. Chamber 634 is formed by the inner wall of bore 104 and the top of piston 648. A manual deadman control switch 635 is held by the operator. To operate the flow control system 100, an operator manually activates a deadman control switch 635 to input pressurized air into cylinder 634. When the operator actuates switch 635, pressurized air is input to air chamber 634 via input port 632. The pressurized air in chamber 634 exerts a force on piston 648 to overcome the force of bias spring 651. As long as the air is applied, and the load sense pressure is below the acceptable threshold pressure, piston 654 of servo 650 remains in the closed position (not shown), blocking passage 126.
A pressure control valve 170 is also included within the housing 102. Pressure control valve 170 includes a piston 178 which is slidably mounted between two bias springs 180 and 182 within a bore 103 of housing 102. A pressure control poppet 174 is attached to piston 178 by a rod 184. Valve 170 has a chamber 183 connected to pressurized chamber 634 by a connecting passage 666, and is thus connected to the pressurized air supply. When the load sense pressure is below a setpoint pressure, poppet 174 is normally open (not shown in FIG. 6).
During normal operation, a gradual increase in the load sense pressure above the setpoint (but below a surge pressure threshold for opening the servo 650) is accommodated by the pressure control valve 170, without affecting the position of surge control servo 650. Normally, when the load sense pressure is below the surge pressure threshold, servo 650 is in the closed position (not shown in FIG. 6), with the bottom of piston 654 blocking flow between passage 126 and main valve outlet 659.
If the load sense pressure in chamber 657 increases above the surge pressure threshold, the sum of the pressure force on the bottom of piston 648 plus the force of spring 651 overcomes the pneumatic pressure force on piston 648. In this state, the surge control servo 650 opens, to the position shown in FIG. 6.
With surge control servo 650 in the open position, the fluid is transmitted rapidly, at a high flow rate, from the inlet 126 of surge control servo 650 to the main valve outlet 659, and into chamber 122. When surge control servo 650 is open, the pressure force on rear face 112b of the flow control valve 110 builds up quickly to close poppet 112, as shown in FIG. 6.
On the other hand, even if the load sense pressure is below the setpoint value, if the operator releases the deadman control switch, the pressurized air supply to chamber 634 is exhausted. With no pressure in chamber 634, the sum of the load sense pressure force on the bottom of piston 648 plus the force of spring 651 is effective to open the surge control servo 650, to the position shown in FIG. 6.
The control servo system 600 of the prior art leaves much to be desired. In the event of differential thermal expansion, fluid may leak from load sense chamber 657 into chamber 634. In that case, the fuel or fluid in chamber 657 mixes into the pressurized air in chamber 634, contaminating the pressurized air supply. Similarly, dirt or o-ring debris may pass from the sides of piston 648 into the pressurized air supply.
Another problem with the system 600 is that the pressurized air in pressure control valve chamber 183 is coupled to the pressurized air supply used by the deadman control 635. Thus, any variations in the pressure of the pressurized air pressure supply also affect the upward pressure exerted on piston 178 of the pressure control valve, and introduce instability into the pressure control. The pressure of the pressurized air supply may vary, and instability in the pressure control is introduced thereby. For example, the air supply pressure may be affected by temperature changes.
Because of these problems with the system as shown in FIG. 6, such systems have been supplanted by a system having an external deadman control loop (not shown). In the external deadman control system, pressurized air is not applied to the pressure control valve 170 or the surge control servo 650. Thus, the position of the surge control servo piston is independent of the deadman control.
The external deadman control system has an additional valve (not shown in FIG. 6) for rerouting fluid in response to the actuation or release of the deadman switch 635. An exemplary valve for performing this function is the "83" series ball valve manufactured by the Whitey Co. of Highland Heights, Ohio. (hereinafter referred to as the "Whitey" valve). In the external deadman system, additional fluid paths (not shown) connect passage 186 and 126 to the "Whitey" valve and connect cavity 176 to the "Whitey" valve. The "Whitey" valve allows fluid to flow from cavity 176 to passage 186 when the handheld deadman switch 625 is activated, and blocks the flow of fluid from passage 126 to cavity 176.
When deadman switch 625 is released, the "Whitey" valve stops the fluid that passes through poppet cavity 176 and passage 186 and returns that fluid with fluid from passage 126 into chamber 122, closing main valve 110. However, the flow rate of fluid through poppet 174 is much less than the flow rate through passage 659 would be if surge control servo is open. As a result, the closing time for the main valve 110 after release of the deadman switch 635 is much slower than the closing time for the main valve 110 after surge control servo 650 moves to the open position shown in FIG. 6. Furthers the external deadman control system for rerouting fluid in response to the actuation or release of the deadman switch 625 is very expensive and slow in closure. An improved servo control system is desired.
FIG. 7 is a cross sectional view of a bypass valve to be used in conjunction with the flow control valve system of FIG. 6. Whereas valve system 600 transmits fuel only when the deadman control 625 is actuated, bypass valve system 700 transmits fuel when the deadman control 625 is released.
The main valve 110 of bypass valve system 700 is similar to the main valve 110 of FIG. 6 described above, and common items in FIGS. 6 and 7 have the same reference numerals. Descriptions of the common components of the flow control valve 600 and the bypass valve 700 are not repeated herein.
Bypass valve system 700 includes a pressure control valve 770. A piston 778 is slidably mounted within a bore 703. Valve 770 has a poppet 772, connected to piston 778 by a rod 784. A biasing spring 782 biases piston 778 towards a closed position. A cylinder 781 between piston 778 arid seal 785 is maintained at the load sense pressure, by a load sense input port 776.
Passage 126 is directly coupled to chamber 122 of the main poppet, and to a chamber 751. When closed, poppet valve 772 separates chamber 751 from a passage 775 that is directly connected to the outlet 124 of main valve 110. When open, poppet valve 772 allows fluid to flow from passage 126 and chamber 751 to passage 775 and outlet 124. Thus, when poppet 772 is closed (not shown in FIG. 7), main poppet 112 is closed (not shown in FIG. 7). When poppet 772 is open (shown in FIG. 7), main poppet 112 is open.
When the load sense pressure exceeds the pressure threshold, pressure in chamber 781 is sufficient so that the sum of the pressure force on piston 778 is sufficient to overcome the bias force of spring 782, opening poppet 772. This in turn releases pressure from chamber 122 so that main valve 112 opens. As a result, the flow from pump 40 is routed through the bypass valve (At the same time, the main valve of flow control system 600 closes, as described above.)
When the manually operated deadman switch 625 is released, the external "Whitey" valve 626 allows fluid in passage 126 to enter passage 752, and flow through passages 786 and 790 to the outlet of bypass valve system 700. This releases pressure from chamber 122, so that the fluid pressure on the front face 112a of main poppet 112 is sufficient to overcome the bias force of spring 118, opening the main valve as shown in FIG. 7.
A pump relief regulator 550 is connected across bypass valve 700, by passages 784 and 788. If the inlet pressure is unacceptably high, regulator 550 allows the fluid to bypass main valve 110 and flow to tank, regardless of whether the deadman control 625 is actuated.
When the operator actuates the deadman switch 625 (position not shown in FIG. 7), "Whitey" valve 626 blocks fluid flow through passage 786; the bypass valve system 700 is normally in the closed state (not shown in FIG. 7). Fluid from the inlet flows from passage 126 into chamber 122 and applies pressure to close piston 112. The fluid flows through flow control valve system 600 (FIG. 6); none of the fluid from the system pump 40 passes through bypass valve system 700 (FIG. 6).
The bypass valve system 700 depends on the flow through passage 786 to release fluid from chamber 122 in order to open the main valve 110. The requirement of the "Whitey" valve is very expensive.
An improved servo control system is desired.